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The Basics of Reverse Osmosis

Osmosis is a naturally occurring phenomenon in which water has a tendency to pass through a semi-permeable membrane from a solution of low total dissolved solids (TDS) into a solution of high total dissolved solids.  Flow of water through the membrane is dependent on the osmotic pressure of the solution, essentially a term describing the magnitude of a solution’s tendency to pass water from one side of the membrane to the other.

A semipermeable membrane, true to its name, is a barrier through which certain components may pass while others are rejected.  In the case of a reverse osmosis membrane, water is freely allowed to pass, while dissolved & suspended contaminants such as dirt, minerals or bacteria are mostly rejected.

RO 1


Semipermeable Membrane

Reverse Osmosis then, is the reversal of the osmosis process: water passes from a solution of high TDS to low TDS.  What this means, is that we can use membranes to remove dissolved solids from water.  So what secret technique is used to reverse the osmosis process?  The secret lies in applied pressure.  A high pressure pump is the driving force behind water production in every RO unit.  There must be enough pressure applied to the feed water in order to overcome the osmotic pressure developed by the dissolved salts present.  The more salts present in the water, the higher the osmotic pressure and the higher the required feed pressure for the RO unit.



Reverse Osmosis

This explains the significant difference between RO units designed for use on city water or surface water, and RO units designed to desalinate seawater.  The dissolved solids in seawater could be 100 times the dissolved solids present in a lake or a river.

The reverse osmosis process is an example of a cross flow filtration system.  Typical media or cartridge filtration uses a full flow design, where all of the feed water passes through the filtration media and is collected as product.  In a cross flow system, only a portion of the feed water is collected as product, and the rest is used to sweep away the dissolved solids that don’t pass through the membrane.  This cross flow prevents premature blockage of the membrane surface and failure of the water system.



Full Flow vs. Cross Flow

RO units then, consist of a number of membrane elements, housed in pressure vessels, a high pressure feed pump, valves, piping and instrumentation to control the various flows, monitor critical parameters and perform basic maintenance functions.  Although units vary in size, materials of construction and controls capabilities, these are the common components found in different designs.



RO Unit – Green = Permeate, Blue = concentrate, Pink = feed/recycle


Various Reverse Osmosis Membranes